This invention relates to a radio communication system comprising a fixed station and a mobile station.
The fixed station has a service area and is assigned a predetermined number of radio channels. At least one mobile station, such as a portable radio telephone set, is movable in the service area. Some of such mobile stations may be carrying out radio communications with the fixed station, through some of the radio channels. When a particular mobile station goes off-hook to request a call service, the fixed station selects one of the vacant radio channels and allots this particular channel to the particular mobile station. Through this channel, the mobile station sends an audio signal to the fixed station in the manner which will presently be described.
Each mobile statio comprises a transmitting circuit and a power unit for controllably supplying electric power to the transmitting circuit and other parts of the mobile station. In the mobile station, the transmitting circuit produces a transmission radio signal on the particular channel with the transmission radio signal modulated by the audio signal.
The power unit is restricted in size and weight. It is therefore desirable to employ a power savings system in the power unit. A typical power saving system is the VOX (voice operated transmitter keyer system). In this connection, it may be mentioned here that the audio signal includes a certain number of pauses. The audio signal therefore comprises an audio signal portion between two adjacent pauses. The audio signal piece will herein be referred to as an audio signal. The VOX system comprises a detector for detecting the audio signal to produce a detection signal which lasts during the presence of the audio signal and consequently has a beginning and an end. The power unit enables the transmitting circuit in response to the detection signal and disables the same during the absence of the detection signal, namely, at each pause.
The fixed station comprises a receiving circuit for receiving the transmission radio signal as a reception radio signal. The receiving circuit holds open the particular channel throughout the aforementioned pauses until the mobile station goes on-hook.
In another conventional radio communication system, the fixed station measures the field intensity of the received radio signal from each radio channel. However, a given mobile station may move far away from the fixed station and the field intensity may be reduced. In the manner described above, a specific channel would continue to be allotted to the such a mobile station. Since the fixed station is provided; with only a limited number of radio channels, it is desirable for effective use of the radio channels to regard a given channel as a vacant radio channel when the field intensity decreases below a predetermined threshold level.
When the last-mentioned system is combined with the VOX system, a problem arises in that the field intensity of the particular channel decreases below the predetermined threshold level at each pause. Thus, the fixed station would regard the channel as vacant despite the fact that the channel should be held open for an audio signal carried on the channel after the pause.